5.1. Semiconductor Detectors 303
energy absorbed in the semiconductor and is highly dependent on the type of radi-
ation and its energy. It should however be mentioned that the atoms displaced by
the incident radiation can also cause further damage. This damage is not a part of
NIEL scaling. However since the energy spectrum of these recoil atoms determines
the type of damage, this effect is not very pronounced for all types of incident radi-
ation. for example charged incident particles generally produce recoil atoms having
a distribution with a long tail at the low energy end, making the secondary point
defect production more probable than the cluster formation. Hence in this case
NIEL scaling will be a good approximation of the overall damage. In the case of
high energy neutrons, however, the recoil spectrum is more skewed towards the high
energy end and therefore NIEL scaling should be interpreted carefully.
Whatever the specific damage mechanism is, the end result is the performance
deterioration of the detector. In the following sections we will discuss the most
important of these deteriorating effects on the detector performance.
L.2 LeakageCurrent
A profound effect of radiation induced damage is the change in reverse bias current,
the obvious cause of which is the increase or decrease in the number of free charge
pairs in the depletion region. The decrease in leakage current, which is generally
observed during initial irradiation, is primarily due to production of charge traps
in the forbidden energy gap. The leakage current can show significant increase
after prolonged irradiation as the probability of charge pair production increases by
introduction of additional energy levels in the forbidden gap.
It has been found that the damage induced leakage current depends on the in-
tegrated radiation dose, the exposed volume of the detector, and its temperature.
The integrated radiation dose, of course, depends on the particle fluence, which is
simply the integrated radiation intensity. At a certain temperature, the dependence
of change in leakage current ilon particle fluence Φ and volumeVcan be written
as (25)
il=αVΦ, (5.1.86)
whereαis the so calleddamage coefficientand depends on the type of incident
particle and its fluence. This equation can also be written in terms of leakage
current before irradiationi 0 and after irradiationir
ir=i 0 +αVΦ. (5.1.87)The temperature dependence ofir, on the other hand, can be described by Boltz-
mann function
ir(T)∝T^2 e−E/^2 kT, (5.1.88)
whereTis the absolute temperature andkis the Boltzmann’s constant. Eis the
activation energy of the material, which is generally higher for irradiated material.
The change in leakage current has unwanted consequences on detector perfor-
mance, most notably, increase in noise and consequent deterioration of signal to
noise ratio. Fortunately the strong dependence on temperature can be easily ex-
ploited to compensate for the deterioration by decreasing the operating tempera-
ture. This is a common practice for detectors used in hostile radiation environments
such as particle accelerators, where radiation induced damage is generally very high.